The present invention relates to solar cells. More specifically, the present invention relates to solar cells with a very thin layer of active material on a surface of a transparent substrate whose opposite surface is grooved at a certain angle, and on which is a layer of reflective material.
In the past it was not the practice in making solar cells of materials like silicon to use thin layers of active material because it resulted in a severe loss in solar cell light absorption.
The loss of light absorption is due to the fact that for the active material, typically silicon, the longer the wavelength of the incident light, the weaker the active material's light absorption. If absorption is weak, the incident light will penetrate through the active material and not be absorbed. To assure absorption of that portion of the solar spectrum consisting of long wavelength radiation, a thick layer of active material was needed. Once the light has been absorbed in the active material, an electron-hole pair is generated. To assure that the lifetime of the generated minority carrier, be it a hole or an electron, was sufficient for the generation of current, the active material had to be of a high quality. In other words, the lifetime of the generated carrier is a dominant factor in determining the diffusion length of carriers generated in the active material. Diffusion length is the average distance a carrier can travel before it recombines. Since a thick layer of active material was required for absorption, a diffusion length at least as great as the active layer thickness was needed to assure current generation by the carriers formed deep in the active layer. Therefore, in the past, a solar cell consisted of a thick layer of highly pure active material.
If a thinner layer of active material could be used, a lower-cost solar cell would result for two reasons. First, a thin layer of active material would greatly reduce the required amount of active material, which is costly. Secondly, a thin layer of active material would require a proportionally shorter diffusion length for optically generated electrons and holes. The ability to use shorter diffusion lengths would allow the use of active material of lower quality, an additional cost saving. Therefore, a much desired need is a solar cell with a thin layer of moderate purity active material with little loss in solar light absorption.